From a DDG search, it looks like the key you need to press is F2, so just
shut the system off, then power it on, and keep pressing F2 until you see the
setup screen.
Another thing you could do is bring up the boot menu with F12 it looks like,
but it looks like you have to enable that in the settings first:
https://ubuntuforums.org/showthread.php?t=2104140&s=8261378ddf5b92927852621ab2bc2fb7&p=12451596#post12451596
Actually, that same thread has another interesting post a couple posts down
from the person who started the thread:
"I finally managed to resolve the problem myself. I knew that it must be
possible to get to the boot menu, because I've seen other posts on linux
forums discussing issues with this particular model of Acer Aspire.
In case anybody else wants to get to the boot menu, you need to hit F2 when
booting the machine. This will initially take you into the very restricted
Setup menu (which doesn't enable you to change boot order). Once in the Setup
menu, hit Esc to exit. The system will try to start Windows, but if you
quickly hit F2 again it takes you into a full Setup menu which enables you to
set the boot order."
